<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 100px;
            background: #58a; clip-path: polygon( 20px 0, calc(100% - 20px) 0,
                                                    100% 20px,100% calc(100% - 20px), 
                                                    calc(100% - 20px) 100%,20px 100%,
                                                     0 calc(100% - 20px), 0 20px );
        }
    </style>
</head>
<body>
    <div class="box"></div>
</body>
</html>